The present invention relates to a device for sealing wrappers in machines for wrapping and/or overwrapping products, in particular packets of cigarettes.
It is a fundamental requirement when heat-sealing thermoplastic materials, or other materials bonded with an interfacing layer of hot melt adhesive, that the elements which perform the sealing action be held in contact with the parts to be sealed for a period of time that remains constant in duration and depends solely upon the nature of the sealed materials.
This unvarying duration of contact must be maintained irrespective of the rate at which products are directed toward and through the sealing station. By the same token, to maintain high levels of productivity, the products must be conveyed at a speed not unduly slowed down by the sealing operation.
The prior art embraces devices equipped with a pair of heated plates positioned on opposite sides of a conveyor belt along which the single products are carried, each enveloped in a respective wrapper. The plates are reciprocated through an operating stroke, which brings them into contact with the end faces of the wrappers, and a corresponding return stroke.
It is absolutely essential with such devices that the duration of contact between the plates and the respective faces of the wrappers not be varied, since the thermoplastic material from which the wrappers are fashioned must be heated up to but not beyond a given temperature, whatever the operating speed of the machine.
The general practice is to connect the two plates to a non-adjustable source of energy, for example to one or more electrical resistors, and then to determine the exact duration of the contact between plates and wrapper employing a variety of means. Given that no wrapping machine will ever operate at one constant speed, it follows that the duration of contact between the heated plates and the wrapper needs to be matched to the different speeds of the machine.
In the device of Italian Patent No. 1,018,092, for example, the duration of contact between the heated plates and the end folds of a wrapper is determined by means of a single cam type element which affords two or more profiles engaged by a following roller associated mechanically with the plates.
Whilst the sealing device embodied in this way has proved capable of effective and precise operation over time, it nonetheless has a marked lack of flexibility inasmuch as a change in the operating speed of the machine dictates the replacement of the cam. This presents obvious drawbacks.
The object of the present invention is to provide a device from which the drawbacks mentioned above are eliminated in a simple and functional manner.